Transmission apparatus and reception apparatus

ABSTRACT

In a multi-antenna communication system using LDPC codes, a simple method is used to effectively improve the received quality by performing a retransmittal of less data without restricting applicable LDPC codes. In a case of a non-retransmittal, a multi-antenna transmitting apparatus transmits, from two antennas, LDPC encoded data formed by LDPC encoding blocks. In a case of a retransmittal, the multi-antenna transmitting apparatus uses a transmission method, in which the diversity gain is higher than in the previous transmission, to transmit only a part of the LDPC encoded data as previously transmitted. For example, the only the part of the LDPC encoded data to be re-transmitted is transmitted from the single antenna.

This is a continuation application of application Ser. No. 15/808,411,filed Nov. 9, 2017, which is a continuation application of applicationSer. No. 14/968,350 filed Dec. 15, 2015, which is a continuationapplication of application Ser. No. 14/244,579 filed Apr. 3, 2014, whichis a continuation application of application Ser. No. 13/566,694 filedAug. 3, 2012, which is a continuation application of application Ser.No. 11/720,046 filed May 23, 2007, which is a national stage ofPCT/JP2005/021154 filed Nov. 17, 2005, which is based on JapaneseApplication No. 2004-340371 filed Nov. 25, 2004, the entire contents ofeach of which are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a retransmission technique in amulti-antenna communication system such as OFDM-MIMO.

BACKGROUND ART

Conventionally, ARQ (Auto Repeat reQuest) is known as a retransmissiontechnique in a radio communication. Furthermore, there is also aproposal of HARQ (Hybrid Auto Repeat reQuest) which combines turbocoding having high error correction performance and ARQ. In HARQ, byusing puncturing processing, it is possible to suppress decrease in datatransmission efficiency caused by retransmission. Such a conventionaltechnique using ARQ and puncturing processing is disclosed in Non-PatentDocument 1 and Non-Patent Document 2.

FIG. 1 shows a configuration example of a conventional transmissionapparatus using ARQ and puncturing processing. Transmission apparatus 20inputs transmission digital signal 1 to CRC (Cyclic Redundancy Check)encoder 2. CRC encoder 2 transmits transmission digital signal 3 afterCRC encoding to convolution code encoder 4. Convolution code encoder 4performs convolution encoding processing on transmission digital signal3 after CRC encoding and transmits transmission digital signal 5 afterconvolution encoding to puncturing section 6.

Puncturing section 6 performs puncturing on transmission digital signal5 after convolution encoding and transmits transmission digital signal 7after puncturing to selection section 12 and also transmits redundantinformation 8 generated upon encoding to storage section 9. Here, when,for example, convolution code encoder 4 performs encoding at a codingrate of ½, puncturing section 6 forms transmission digital signal 7after puncturing, for example, at a coding rate of ¾.

Redundant information 8 stored in storage section 9 is discarded when anACK (acknowledgement) signal is transmitted as ACK/NACK signal 11 from acommunicating party, and transmitted to selection section 12 as aretransmission signal when a NACK (negative-acknowledgement) signal istransmitted.

Selection section 12 selects and outputs transmission digital signal 7after puncturing when an ACK signal is inputted, and, on the other hand,selects and outputs redundant information 10 stored in storage section 9when a NACK signal is inputted. That is, selection section 12 selectsredundant information 10 as a retransmission signal only when there is aretransmission request.

Modulation section 14 performs modulation such as QPSK and 16QAM ontransmission digital signal 13 selected by selection section 12 andtransmits modulated signal 15 obtained in this way to radio section (RFsection) 16. RF section 16 performs predetermined radio processing suchas frequency conversion on modulated signal 15 and transmitstransmission signal 17 obtained in this way to antenna 18.

-   Non-Patent Document 1: “Rate-compatible punctured convolutional    codes and their applications,” IEEE Transactions on Communications,    pp. 389-400, vol. 36, no. 4, April 1988.-   Non-Patent Document 2: “Generalized Type II Hybrid ARQ scheme using    punctured convolutional coding” IEEE Transactions on Communications,    pp. 1938-1946, vol. 38, no. 11, November 1990.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

By the way, LDPC (Low Density Parity Check) coding attracts attention asa coding scheme capable of obtaining excellent coding gain such ashaving performance close to a Shannon limit, good block error rateperformance and producing substantially no error floor.

However, when an attempt is made to perform ARQ using an LDPC codeinstead of a convolution code such as a turbo code, the followingproblem occurs. That is, when an attempt is made to realize ARQ bycombining an LDPC code and puncturing processing, LDPC codes capable ofperforming appropriate puncturing processing are quite limited.Therefore, there is a problem that the flexibility of the design isreduced. Furthermore, when all data transmitted last time isretransmitted without performing puncturing processing, the amount ofretransmission data increases, and such an attempt is not realistic.

Therefore, it is necessary to realize new ARQ which does not requirepuncturing processing. At this time, it is expected that only a smallnumber of retransmission times is required and the method thereof issimple.

The present invention has been implemented in view of theabove-described problems, and it is therefore an object of the presentinvention to provide a multi-antenna transmission apparatus,multi-antenna reception apparatus and data retransmission method capableof effectively improving received quality through retransmission using asimple method without limiting applicable LDPC codes and with lessretransmission data in a multi-antenna communication system using LDPCcodes.

Means for Solving the Problem

In order to solve the above-described problems, the present inventionadopts a configuration including: an LDPC encoding section that encodestransmission data using an LDPC code; a transmission section thattransmits the LDPC encoded transmission data assigned to a plurality ofantennas; and a transmission control section that controls transmissionso that, upon retransmission, only part of LDPC encoded data out of theLDPC encoded data transmitted last time is transmitted using atransmission method having a higher diversity gain than the lasttransmission.

According to this configuration, upon retransmission, only part of LDPCencoded data out of the LDPC encoded data transmitted last time istransmitted using a transmission method having a higher diversity gainthan the last transmission, so that it is possible to receive part ofthe retransmitted LDPC code in a high quality condition. Here, the LDPCcode has a much longer constraint length than that of a convolutioncode, and therefore, when the quality of part of the data is good, theLDPC code has a characteristic that, upon decoding, the error rate ofthe other decoded data is improved accompanying the part of the datahaving good quality, so that the overall error rate performance of thedecoded data is improved. As a result, upon retransmission, it ispossible to largely improve the error rate performance of received datathrough retransmission with less retransmission data than a convolutioncode or the like without deteriorating the flexibility of the LDPC codesince puncturing is not performed.

Advantageous Effect of the Invention

As described above, according to the present invention, in amulti-antenna communication system using an LDPC code, it is possible torealize a multi-antenna transmission apparatus, multi-antenna receptionapparatus and data retransmission method capable of effectivelyimproving received quality through retransmission using a simple method,without limiting the applicable LDPC code and with less retransmissiondata.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing the configuration of the conventionaltransmission apparatus which performs ARQ;

FIG. 2 is a block diagram showing the configuration of a multi-antennatransmission apparatus according to Embodiment 1 of the presentinvention;

FIG. 3A and FIG. 3B show a frame configuration example of the modulatedsignals transmitted from antennas when a multi-antenna transmissionapparatus carries out normal transmission (that is, transmission otherthan retransmission);

FIG. 4 is a block diagram showing the configuration of a multi-antennareception apparatus according to Embodiment 1;

FIG. 5 shows an image of a multi-antenna communication system;

FIG. 6 is a block diagram showing the configuration of the decodingsection;

FIG. 7 is a data flow chart illustrating the operation of theembodiment;

FIG. 8A and FIG. 8B illustrate the operation of the LDPC coding section,FIG. 8A shows input/output data of the LDPC encoding section, and FIG.8B shows contents of retransmission data;

FIG. 9 shows an image of a system for maximum ratio combining;

FIG. 10 illustrates a space-time block code;

FIG. 11 is a data flow chart in the case of retransmittingretransmission data using the space-time block code;

FIG. 12 is a block diagram showing the configuration of a multi-antennatransmission apparatus which retransmits retransmission data using thespace-time block code;

FIG. 13 illustrates Cyclic Delay Diversity;

FIG. 14 shows an image of a multi-antenna communication system whenthere are three transmit antennas and three receive antennas; and

FIG. 15 shows an image of a multi-antenna communication system whenthere are two transmit antennas and three receive antennas.

BEST MODE FOR CARRYING OUT THE INVENTION

The inventors of the present invention have focused on the fact that anLDPC code has a longer constraint length than a convolution code such asa turbo code and has a characteristic that, when received quality ofpart of data is good, the error rate characteristic of the other dataalso improves accompanying the part of the data having good quality. Theinventors have also considered that, upon retransmission of the LDPCcode, if only part of data is retransmitted with sufficient quality, itis possible to obtain sufficient effects of improving the error ratecharacteristic through retransmission using less retransmission dataeven if puncturing is not carried out.

Features of the present invention include transmitting, uponretransmission, only part of LDPC encoded data transmitted last timeusing a transmission method having a higher diversity gain than upon thelast transmission. By this means, it is possible to effectively improvereceived quality through retransmission with less retransmission dataeven if puncturing is not carried out.

Embodiment

Hereinafter, an embodiment of the present invention will be explained indetail with reference to the accompanying drawings.

FIG. 2 shows the configuration of a multi-antenna transmission apparatusaccording to the embodiment of the present invention. Multi-antennatransmission apparatus 100 is provided, for example, to a radio basestation. Multi-antenna transmission apparatus 100 is a transmissionapparatus which performs so-called OFDM-MIMO communication and designedto transmit different modulated signals from two antennas. Morespecifically, multi-antenna transmission apparatus 100 transmitsmodulated signal A from antenna 114A and transmits modulated signal Bfrom antenna 114B. Here, in FIG. 2, the signal processing systemrelating to modulated signal A and the signal processing system relatingto modulated signal B have substantially similar configuration, andtherefore the processing system of modulated signal A is indicated as“A” appended to reference numerals, and the processing system ofmodulated signal B which corresponds to the signal processing systemrelating to modulated signal A is indicated as “B” appended to referencenumerals.

Frame configuration signal generation section 116 of multi-antennatransmission apparatus 100 functions as a transmission control section,determines a transmission frame configuration based on ACK/NACK signal115 which is retransmission request information transmitted from thecommunicating party (here, the communicating party is named a“terminal”) and transmits the determined transmission frameconfiguration to LDPC encoding sections 102A and 102B and modulationsections 106A and 106B as frame configuration signal 117.

LDPC encoding sections 102A and 102B receive transmission digitalsignals 101A and 101B of modulated signal A and frame configurationsignal 117 as input, perform CRC (Cyclic Redundancy Check) encoding andLDPC encoding on transmission digital signals 101A and 101B and transmittransmission digital signals 103A and 103B of encoded modulated signalsA and B to modulation sections 106A and 106B. Furthermore, transmissiondigital signals 103A and 103B of encoded modulated signals A and B arestored in storage sections 104A and 104B, respectively. The data storedin storage sections 104A and 104B are used for retransmission.

Modulation sections 106A and 106B perform modulation processing such asQPSK (Quadrature Phase Shift Keying) and 16QAM. Modulation section 106Amodulates any one of transmission digital signals 103A, 105A and 105Bbased on frame configuration signal 117. More specifically, whentransmission is not retransmission, transmission digital signal 103A ofencoded modulated signal A is modulated and outputted, and, when thereis a request for retransmission of modulated signal A, transmissiondigital signal 105A of encoded modulated signal A which is stored instorage section 104A is modulated and outputted, and, when there is arequest for retransmission of modulated signal B, transmission digitalsignal 105B of encoded modulated signal B which is stored in storagesection 104B is modulated and outputted.

On the other hand, modulation section 106B modulates transmissiondigital signal 103B based on frame configuration signal 117. Morespecifically, when retransmission does not take place, transmissiondigital signal 103B of encoded modulated signal B is modulated andoutputted, and nothing is outputted upon retransmission.

In this way, while different modulated signals A and B are transmittedfrom two antennas 114A and 114B during normal transmission, a modulatedsignal whose retransmission is requested is transmitted from only oneantenna 114A upon retransmission. As a result, upon retransmission, themodulated signal transmitted from only one antenna 114A is received by aplurality of antennas on the reception side, so that it is possible toperform transmission with a high diversity gain. Therefore, the receivedquality upon retransmission can be improved compared to normaltransmission.

Modulated signals 107A and 107B outputted from modulation sections 106Aand 106B are converted to parallel signals 109A and 109B byserial/parallel conversion sections (S/P) 108A and 108B, respectively,subjected to inverse Fourier transform by inverse Fourier transformsections (idft) 110A and 110B that follow, resulting in OFDM signals111A and 111B. OFDM signals 111A and 111B are subjected to predeterminedradio processing such as frequency conversion by radio sections 112A and112B, resulting in transmission signals 113A and 113B and thentransmitted from antennas 114A and 114B. In this embodiment, it isassumed that the signal transmitted from antenna 114A is called“modulated signal A”, and the signal transmitted from antenna 114B iscalled “modulated signal B” in principle.

FIG. 3A and FIG. 3B shows a frame configuration example of the modulatedsignals transmitted from antennas 114A and 114B when multi-antennatransmission apparatus 100 carries out normal transmission (that is,transmission other than retransmission). As shown in FIG. 3A and FIG.3B, multi-antenna transmission apparatus 100 transmits modulated signalA and modulated signal B simultaneously from different antennas. In FIG.3A and FIG. 3B, control information symbol 201 is a symbol to transmitcontrol information such as a frame configuration, information as towhether or not data is retransmission data, and the number ofretransmission times. Pilot symbol 202 is a symbol to estimate channelcondition on the reception side. Data symbol 203 is CRC encoded and LDPCencoded data. The signal of modulated signal A in the frameconfiguration shown in FIG. 3A is transmitted from antenna 114A and thesignal of modulated signal B in the frame configuration shown in FIG. 3Bis transmitted from antenna 114B. Furthermore, it is assumed thatsymbols of the same carrier and the same time are transmittedsimultaneously.

FIG. 4 shows the configuration of a multi-antenna reception apparatus ofthis embodiment. Multi-antenna reception apparatus 300 is provided to aterminal which communicates with the base station to which multi-antennatransmission apparatus 100 is provided.

Furthermore, FIG. 5 shows the overall configuration of a multi-antennacommunication system according to this embodiment. First, beforeexplaining the configuration of multi-antenna reception apparatus 300 inFIG. 4, the overall operation of multi-antenna communication system 400in FIG. 5 will be explained.

Multi-antenna communication system 400 has multi-antenna transmissionapparatus 100 in FIG. 2 and multi-antenna reception apparatus 300 inFIG. 4. In multi-antenna transmission apparatus 100, transmissionsection 401 (equivalent to the part excluding antennas 114A and 114B inFIG. 2) forms modulated signal A (113A) and modulated signal B (113B)from transmission digital signals 101A and 101B and transmits modulatedsignal A (113A) and modulated signal B (113B) from antennas 114A and114B. In FIG. 5, modulated signal A (113A) is indicated as Txa(t), andmodulated signal B (113B) is indicated as Txb(t).

Multi-antenna reception apparatus 300 inputs received signals 302X and302Y received at antennas 301X and 301Y (indicated as R1(t) and R2(t) inFIG. 5) to reception section 402 (equivalent to the part excludingantennas 301X and 301Y in FIG. 4). Reception section 402 performsdemodulation processing on received signals R1(t) and R2(t) and therebyobtains received data 316A and 316B which correspond to transmissiondigital signals 101A and 101B. Furthermore, reception section 402obtains ACK/NACK signals 317A and 317B which correspond to transmissiondigital signals 101A and 101B.

Here, modulated signal Txa(t) transmitted from antenna 114A is subjectedto channel fluctuations h12(t) and h12(t) and received at antennas 301Xand 301Y. On the other hand, modulated signal Txb(t) transmitted fromantenna 114B is subjected to channel fluctuations h21(t) and h22(t) andreceived at antennas 301X and 301Y.

Therefore, the following relational expression holds.

$\begin{matrix}\lbrack 1\rbrack & \; \\{\begin{pmatrix}{R\; 1(t)} \\{R\; 2(t)}\end{pmatrix} = {\begin{pmatrix}{h\; 11(t)} & {h\; 21(t)} \\{h\; 12(t)} & {h\; 22(t)}\end{pmatrix}\begin{pmatrix}{{Txa}(t)} \\{{Txb}(t)}\end{pmatrix}}} & ( {{Equation}\mspace{14mu} 1} )\end{matrix}$

Next, the configuration of multi-antenna reception apparatus 300 of thisembodiment shown in FIG. 4 will be explained. Multi-antenna receptionapparatus 300 inputs received signals 302X and 302Y received at antennas301X and 301Y to radio sections 303X and 303Y. Radio sections 303X and303Y perform predetermined radio processing such as frequency conversionon received signals 302X and 302Y, respectively, thereby obtainreception baseband signals 304X and 304Y and transmit reception basebandsignals 304X and 304Y to Fourier transform sections (dft) 305X and 305Y.

Fourier transform sections 305X and 305Y perform Fourier transformprocessing on reception baseband signals 304X and 304Y, respectively andoutput signals 306X and 306Y after the Fourier transform.

Channel condition estimation section 307X of modulated signal A hassignal 306X after the Fourier transform as input, detects pilot symbol202 of modulated signal A in FIG. 3A and FIG. 3B, estimates channelfluctuation of modulated signal A based on pilot symbol 202 of modulatedsignal A, that is, estimates h12(t) in equation 1. At this time, channelcondition estimation section 307X of modulated signal A estimates thechannel fluctuation of modulated signal A for each carrier. Then,channel condition estimation section 307X of modulated signal A outputschannel estimation signal 308X for each carrier of modulated signal A.

Channel condition estimation section 309X of modulated signal B hassignal 306X after the Fourier transform as input, detects pilot symbol202 of modulated signal B in FIG. 3A and FIG. 3B, estimates channelfluctuation of modulated signal B based on pilot symbol 202 of modulatedsignal B, that is, estimates h12(t) of equation 1. At this time, channelcondition estimation section 309X of modulated signal B estimateschannel fluctuation of modulated signal B for each carrier. Then,channel condition estimation section 309X of modulated signal B outputschannel estimation signal 310X for each carrier of modulated signal B.

Channel condition estimation section 307Y of modulated signal A hassignal 306Y after the Fourier transform as input, detects pilot symbol202 of modulated signal A in FIG. 3A and FIG. 3B, estimates channelfluctuation of modulated signal A based on this, that is, estimatesh21(t) of equation 1. At this time, channel condition estimation section307Y of modulated signal A estimates channel fluctuation of modulatedsignal A for each carrier. Then, channel condition estimation section307Y of modulated signal A outputs channel estimation signal 308Y foreach carrier of modulated signal A.

Channel condition estimation section 309Y of modulated signal B hassignal 306Y after the Fourier transform as input, detects pilot symbol202 of modulated signal B in FIG. 3A and FIG. 3B, estimates channelfluctuation of modulated signal B based on pilot symbol 202 of modulatedsignal B, that is, estimates h22(t) of equation 1. At this time, channelcondition estimation section 309Y of modulated signal B estimateschannel fluctuation of modulated signal B for each carrier. Then,channel condition estimation section 309Y of modulated signal B outputschannel estimation signal 310Y for each carrier of modulated signal B.

Control information detection section 318 has signals 306X and 306Yafter the Fourier transform as input, detects control information symbol201 in FIG. 3A and FIG. 3B, obtains control information 319 whichconsists of information of the frame configuration, information as towhether or not data is retransmission data, information of the number ofretransmission times based on control information symbol 201 and outputscontrol information 319 to signal processing section 311 and decodingsections 313A and 313B.

Signal processing section 311 has channel estimation signals 308X and308Y of modulated signal A, channel estimation signals 310X and 310Y ofmodulated signal B, signals 306X and 306Y after the Fourier transform,control information 319 as input, and, based on the frame configurationand information whether or not the data is retransmission data indicatedby control information 319, separates, when the data is notretransmission data, each of modulated signals A and B by performinginverse matrix calculation in equation 1 and outputs the baseband signalof modulated signal A as baseband signal 312A and the baseband signal ofmodulated signal B as baseband signal 312B. On the other hand, in thecase of retransmission data, signal processing section 311 performsmaximum ratio combining, signal processing for a space-time block codeor signal processing for Cyclic Delay Diversity and outputs basebandsignal 312A and/or baseband signal 312B.

Decoding section 313A has baseband signal 312A and control information319 as input, decodes modulated signal A based on the information as towhether or not the data is retransmission data in control information319 and outputs digital signal 314A after decoding modulated signal A.At this time, when control information 319 indicates that thetransmission is not retransmission, decoding section 313A performsnormal LDPC decoding processing. On the other hand, when controlinformation 319 indicates that the transmission is retransmission,decoding section 313A replaces the LDPC encoded data which correspondsto the retransmitted part of LDPC encoded data of the LDPC encoded datareceived last time (that is, the stored LDPC encoded data) byretransmitted LDPC encoded data and performs LDPC decoding processing.

CRC checking section 315A has digital signal 314A after decodingmodulated signal A as input, carries out CRC check and detects asituation of error occurrence. Next, in the case where no error hasoccurred, CRC checking section 315A outputs received data 316A ofmodulated signal A and forms an ACK signal as an ACK/NACK signal 317A tobe transmitted to the communicating party. On the other hand, in thecase where an error has occurred, CRC checking section 315A forms a NACKsignal as ACK/NACK signal 317A to be transmitted to the communicatingparty without outputting received data.

In the same way, decoding section 313B has baseband signal 312B andcontrol information 319 as input, decodes modulated signal B based onthe information as to whether or not the data is retransmission data incontrol information 319 and outputs digital signal 314B after decodingmodulated signal B. At this time, when control information 319 indicatesthat the transmission is not retransmission, decoding section 313Bperforms normal LDPC decoding processing. On the other hand, whencontrol information 319 indicates that the transmission isretransmission, decoding section 313B replaces the LDPC encoded datawhich corresponds to the retransmitted part of LDPC encoded data out ofthe LDPC encoded data received last time (that is, the stored LDPCencoded data) by the retransmitted LDPC encoded data and performs LDPCdecoding processing.

CRC checking section 315B has digital signal 314B after decodingmodulated signal B as input, performs a CRC check, detects a situationof error occurrence and outputs, when no error has occurred, receiveddata 316B of modulated signal B and forms an ACK signal as ACK/NACKsignal 317B to be transmitted to the communicating party. On the otherhand, when an error has occurred, CRC checking section 315B forms a NACKsignal as ACK/NACK signal 317B to be transmitted to the communicatingparty without outputting received data.

FIG. 6 shows a configuration example of decoding section 313A (313B).Decoding section 313A and decoding section 313B have a similarconfiguration, and therefore only the configuration of decoding section313A will be explained here.

Decoding section 313A has LDPC decoding section 501 and storage section502. Storage section 502 stores baseband signal 312A received until thelast time. When control information 319 indicates that the transmissionis not retransmission, LDPC decoding section 501 performs LDPC decodingprocessing using the only baseband signal 312A received this time andobtains digital signal 314A after decoding. On the other hand, whencontrol information 319 indicates that the transmission isretransmission, LDPC decoding section 501 performs LDPC decodingprocessing using baseband signal 503 stored until the last time instorage section 502 and baseband signal 312A retransmitted this time. Atthis time, LDPC decoding section 501 replaces the signal whichcorresponds to retransmitted baseband signal 312A (retransmitted part ofLDPC encoded data) out of baseband signal 503 received until the lasttime stored in storage section 502 by retransmitted baseband signal 312Aand performs LDPC decoding processing.

Next, the operation of this embodiment will be explained. First, anexample of the overall flow of transmission/reception betweenmulti-antenna transmission apparatus 100 (hereinafter, referred to as“base station”) and multi-antenna reception apparatus 300 (hereinafter,referred to as “terminal”) is shown in FIG. 7.

First, the base station transmits data 1A with modulated signal A anddata 1B with modulated signal B as shown in FIG. 7 <1>.

The terminal receives this signal, but, when no error has occurred, theterminal does not transmit any retransmission request to the basestation as shown in FIG. 7 <2>. The base station then transmits new data2A with modulated signal A and new data 2B with modulated signal B asshown in FIG. 7 <3>.

The terminal receives this signal, and, when an error has occurred, theterminal transmits a retransmission request to the base station as shownin FIG. 7 <4>. The base station then retransmits only part of data(hereinafter, such data will be referred to as “partial data”) 2A and P1out of data 2A transmitted last time, and partial data 2B and P1 out ofdata 2B transmitted last time as shown in FIG. 7 <5>.

The terminal performs LDPC decoding using the received signals of data2A and data 2B transmitted by the base station in FIG. 7 <3> and thereceived signals of partial data 2A and P1 of modulated signal A andpartial data 2B and P1 of modulated signal B transmitted in FIG. 7 <5>.As a result, when no error occurs, as shown in FIG. 7 <6>, noretransmission request is transmitted to the base station. The basestation then transmits new data 3A with modulated signal A and new data3B with modulated signal B as shown in FIG. 7 <7>.

The terminal receives this signal, and, when an error has occurred, theterminal transmits a retransmission request to the base station as shownin FIG. 7 <8>. The base station then retransmits only partial data of 3Aand P1 of data 3A transmitted last time and partial data 3B and P1 ofdata 3B transmitted last time as shown in FIG. 7 <9>.

The terminal performs LDPC decoding using the received signals of data3A and data 3B transmitted by the base station in FIG. 7 <7> and thereceived signals of partial data 3A and P1 of modulated signal A andpartial data 3B and P1 of modulated signal B transmitted in FIG. 7 <9>.As a result, when an error still occurs, a retransmission request istransmitted again to the base station as shown in FIG. 7 <10>.

The base station then retransmits the only partial data 3A and P2 ofdata 3A and partial data 3B and P2 of data 3B as shown in FIG. 7 <11>.Partial data 3A and P2 transmitted through this second retransmission isdata which is different from partial data 3A and P1 at the firstretransmission. In the same way, partial data 3B and P2 transmittedthrough the second retransmission is data which is different frompartial data 3B and P1 transmitted through the first retransmission.

The terminal performs LDPC decoding using the received signals of data3A and data 3B transmitted by the base station in FIG. 7 <7>, thereceived signals of partial data 3A and P1 and partial data 3B and P1transmitted in FIG. 7 <9> and partial data 3A and P2 and partial data 3Band P2 transmitted in FIG. 7 <11>. As a result, when no error occurs, noretransmission request is transmitted to the base station as shown inFIG. 7 <12>.

Next, the operations of LDPC encoding sections 102A and 102B will beexplained in detail using FIG. 8A and FIG. 8B. However, to simplifyexplanations here, CRC encoding will be omitted. Furthermore, especiallyas shown in FIG. 7, the cases will be described as examples where data3A is transmitted first, partial data 3A and P1 are retransmitted first,and partial data 3A and P2 are retransmitted at the second time andwhere data 3B is transmitted first, partial data 3B and P1 areretransmitted first and partial data 3B and P2 are retransmitted at thesecond time.

LDPC encoding section 102A for modulated signal A outputs LDPC encodeddata 103A by performing LDPC encoding on transmission digital signal101A before encoding. For example, assuming that transmission digitalsignal 101A is (m1 a, m2 a, . . . , m1603 a) and a parity check matrixis G, (C1 a, C2 a, . . . , C2000 a) is outputted as LDPC encoded data103A.

LDPC encoding section 102B for modulated signal B performs LDPC encodingon transmission digital signal 101B before encoding and thereby outputsLDPC encoded data 103B. For example, assuming that transmission digitalsignal 101B is (m1 b, m2 b, . . . , m1603 b) and a parity check matrixis G, (C1 b, C2 b, . . . , C2000 b) is outputted as LDPC encoded data103B.

Upon first transmission, the LDPC encoded data (C1 a, C2 a, . . . ,C2000 a), (C1 b, C2 b, . . . , C2000 b) generated in this way aretransmitted as data 3A and 3B from different antennas 114A and 114B.Furthermore, part or all of LDPC encoded data (C1 a, C2 a, . . . , C2000a) is stored in storage section 104A and part or all of LDPC encodeddata (C1 b, C2 b, . . . , C2000 b) is stored in storage section 104B.

Then, upon first retransmission, part of LDPC encoded data (C101 a, C102a, . . . , C600 a) stored in storage section 104A is transmitted aspartial data 3A and P1 as shown in FIG. 8B, and part of LDPC encodeddata (C101 b, C102 b, . . . , C600 b) stored in storage section 104B istransmitted as partial data 3B and P1. Here, in this embodiment, uponretransmission of the partial data, the partial data is transmitted onlyfrom one antenna 114A.

Upon second retransmission as shown in FIG. 8B, the LDPC encoded data(C901 a, C902 a, . . . , C1400 a) of the LDPC encoded data stored instorage section 104A, which is different from the LDPC encoded dataretransmitted at the first time is transmitted as partial data 3A andP2, and the LDPC encoded data (C901 b, C902 b, . . . , C1400 b) of theLDPC encoded data stored in storage section 104B, which is differentfrom the LDPC encoded data retransmitted at the first time istransmitted as partial data 3B and P2. When the partial data isretransmitted, the data is also transmitted only from one antenna 114A.

When such retransmission is performed, the processing of decodingsection 313A shown in FIG. 6 will be explained. First, when data 3A and3B are transmitted at the first transmission (FIG. 7 <7>), LDPC decodingsection 501 performs LDPC decoding on data 3A and 3B and stores data 3Aand 3B in storage section 502. Upon first retransmission (FIG. 7 <9>),LDPC decoding section 501 performs LDPC decoding processing usingretransmitted partial data 3A and P1, partial data 3B and P1, and data3A and 3B stored in storage section 502 and also stores retransmittedpartial data 3A and P1, and partial data 3B and P1 in storage section502. Upon second retransmission (FIG. 7 <11>), LDPC decoding section 501performs LDPC decoding processing using retransmitted partial data 3Aand P2, partial data 3B and P2, and data 3A, 3B, 3A, P1, 3B and P1stored in storage section 502.

Next, the reason will be explained why, by transmitting a retransmissionsignal only from one antenna 114A upon retransmission as in thisembodiment, a greater diversity gain than that upon normal transmissioncan be obtained, and received quality of retransmission data (partialdata) can be improved.

FIG. 9 shows an image when transmitting retransmission data with thesame reference numerals assigned to the corresponding components in FIG.5. Multi-antenna transmission apparatus 100 transmits the modulatedsignal only from antenna 114A. The modulated signal transmitted fromantenna 114A is multiplied by propagation coefficients of h11(t) andh12(t) on the propagation path and received at receive antennas 301X and301Y. Therefore, reception section 402 of multi-antenna receptionapparatus 300 can combine the modulated signals transmitted from antenna114 at a maximum ratio, so that received quality of the retransmitteddata improves.

By the way, data 3A and data 3B transmitted from different antennas inFIG. 7 <7> have poor received quality, and therefore the terminaltransmits a retransmission request as shown in FIG. 7 <8>. At this time,in an environment in which no considerable change occurs in thepropagation environment, when retransmission data is also transmittedafter being subjected to MIMO multiplexing in the same way as in FIG. 7<7>, it leads to a little improvement effect in received quality, butmay not lead to a considerably large improvement effect.

On the other hand, in this embodiment, by transmitting partial data ofthe LDPC encoded data from fewer antennas upon retransmission than uponthe last transmission, a greater diversity gain can be obtained on thereception side, so that it is possible to improve received quality ofthe retransmitted partial data even in an environment in which noconsiderable change occurs in the propagation environment and obtain aconsiderable improvement effect in received quality throughretransmission.

In addition, by decreasing the M-ary number upon retransmission,received quality can be further improved, so that it is possible toobtain a further greater improvement effect in received quality throughretransmission.

Here, the difference between the transmission method of the presentinvention and the conventional ARQ is that data 3A and P1, data 3A andP2, data 3B and P1, and data 3B and P2 retransmitted in this embodimentare not the data obtained through puncturing, but merely partial data(part of the data transmitted last time). In this way, since nopuncturing processing is performed in this embodiment, this embodimentcan be applied to any LDPC code. That is, the LDPC code is not limitedby puncturing processing.

Furthermore, in the conventional ARQ using puncturing, data to beretransmitted is limited to redundant information generated uponpuncturing (that is, generated upon encoding), but the retransmissionmethod of this embodiment is never limited to this, and therefore it ispossible to perform quite flexible retransmission. For example, it ispossible to perform processing such as receiving information equivalentto the received quality simultaneously with an ACK/NACK signal from thecommunicating party and changing the amount of partial data to beretransmitted, and also adopt an adaptive retransmission method such aschanging the amount of the retransmission data according to acommunication situation.

Here, in this embodiment, the reason will be explained why it ispossible to effectively reduce errors in received data through simpleretransmission processing such as retransmitting only partial datawithout performing any puncturing processing.

For example, when using a convolution code, partial data isretransmitted without performing puncturing as in the case of thisembodiment. A convolution code has quite a short constraint length thanthat of an LDPC code. The convolution code generally uses a constraintlength of approximately 10. Therefore, even if partial data of goodquality is transmitted, the influence thereof remains within the rangeof the constraint length. Therefore, in order to improve the overallerror rate performance of the data block, it is necessary to transmit alarge amount of partial data. Therefore, it is possible to say that themethod of retransmitting the partial data as described in thisembodiment is not so suitable for the case where a convolution code isused.

On the other hand, as this embodiment, when partial data isretransmitted without puncturing when an LDPC code is used, it ispossible to obtain remarkable effects. This results from the fact thatthe constraint length of the LDPC code is much longer than that of theconvolution code. This will be explained using FIG. 8A and FIG. 8B. Forexample, transmission digital signal m1 a on the reception side isestimated using some of information of the LDPC encoded data (C1 a, C2a, . . . , C2000 a) generated using parity check matrix G. When thereceived quality of part of some of the information is improved, thereceived quality (that is, an error rate characteristic) of transmissiondigital signal m1 a is improved. This is not limited to transmissiondigital signal m1 a, but the same also applies to transmission digitalsignals m2 a to m1603 a. That is, since the LDPC code has a longconstraint length, by improving the error rate performance of partialdata, it is also possible to improve the error rate characteristic ofmany other data within the constraint length. By this means, it ispossible to effectively improve the error rate characteristic withoutthe LDPC code being limited by puncturing processing and with a smallamount of retransmission.

Next, a preferable example of the method of selecting partial data to beretransmitted (hereinafter, referred to as “partial data”) will beexplained.

It is assumed that message m=(m₁, . . . , m_(i)), transmission sequenceX=(x₁, . . . , x_(n))^(T) and reception sequence Y=(y₁, . . . ,y_(n))^(T). Here, m<n, ^(T) indicates transposition. When it is assumedthat a generator matrix is G and an check matrix is H, the relationshipin the following expression holds.[2]GH ^(T)=0  (Equation 2)

In LDPC encoding, transmission sequence X is generated from message mand generator matrix G. On the decoding side, decoding is performedusing reception sequence Y and check matrix H.

Here, assuming that K pieces of partial data are retransmitted, themethod of selecting the K pieces of data will be explained.

Attention will be focused on a vector of column p and a vector of columnq of check matrix H. It is assumed that the number of “1”s of the vectorof column p is P and the number of “1”s of the vector of column q is Q.Here, it is assumed that P>Q. In this case, when transmission sequencexp is compared with transmission sequence xq, transmission sequence xphas stronger resistance to noise than transmission sequence xq. Whenthis is taken into consideration, as partial data to be retransmitted,it is preferable to select transmission sequence xp which has more “1”scompared to selecting transmission sequence xq in a column vector of ancheck matrix.

The present application adopts the above-described concept. Morespecifically, top K (K<n) column vectors having many “1”s of a vector ofcolumn j (1<j<n) of check matrix H are searched. Here, it is assumedthat a set of top K column vectors having many “1”s is (r1, . . . , rK).Assuming that the transmission bit obtained using column vector r1 isx_(r1), . . . , the transmission bit obtained using column vector rK isx_(rK), transmission bits (x_(r1), . . . , x_(rK)) are selected as thepartial data to be retransmitted. That is, K-bit data obtained using topK (1<K<n) column vectors which include more “1”s out of n column vectorsof check matrix H is retransmitted as the partial LDPC encoded data. Bythis means, data having strong resistance to error can be used asretransmission data, so that it is possible to expect a furtherimprovement effect in received quality through retransmission.

Here, as the above-described method of transmitting retransmission data,the following two methods can be considered.

First, transmission bits (x_(r1), . . . , x_(rK)) are always assumed tobe retransmission data. That is, also upon retransmission of the secondtime or later, transmission bits (x_(r1), . . . , x_(rK)) aretransmitted.

Second, the data to be retransmitted is changed according to the numberof retransmission times. That is, transmission bits (x_(r1), . . . ,x_(rK)) are transmitted at the first retransmission. At the secondretransmission, the set of top K column vectors (r1, . . . , rK) havingmost “1”s is excluded, and transmission bits (x_(s1), . . . , x_(sK))which correspond to a set (s1, . . . , sK) of top K column vectorshaving the second most “1”s is assumed to be the retransmission data.The retransmission of the third time or later will also be considered ina similar way.

Furthermore, when the above-described retransmission is performed, ifthe LDPC code to be used is determined, the information of the columnvector used for retransmission can be shared on the transmission sideand the reception side, so that it is possible to readily decode theretransmitted partial bits on the reception side.

Thus, according to this embodiment, at multi-antenna transmissionapparatus 100 which transmits LDPC encoded data from a plurality ofantennas, upon retransmission, by transmitting only part of the LDPCencoded data out of the LDPC encoded data transmitted last time using atransmission method having a higher diversity gain than upon the lasttransmission, it is possible to realize ARQ having a greater improvementeffect in received quality through retransmission using a simple method,without limiting applicable LDPC codes and with less retransmissiondata.

Furthermore, part of LDPC encoded data can be retransmitted withoutbeing restricted by the puncturing processing, so that it is alsopossible to realize extremely flexible retransmission such as changingthe amount of data adaptively.

Furthermore, when retransmission is performed a plurality of times, asshown in FIG. 8B, for example, part of LDPC encoded data to beretransmitted is changed for every retransmission, so that it ispossible to further enhance the improvement effect in received qualitythrough retransmission.

In the above-described embodiment, the case has been described where thepartial data as shown in FIG. 8B is retransmitted, but the partial datato be retransmitted is not limited to the example shown in FIG. 8B. Itis more preferable to select partial data to be retransmitted alsotaking into consideration fluctuation in the propagation environment.The propagation environment generally changes gradually in the timedirection and in the frequency direction. Therefore, when the partialdata to be transmitted is comprised of only symbols which exist in abiased frequency band or symbols which exist in a biased time, there isa high possibility that the diversity effect may decrease. Consideringthe diversity effect, partial data is formed by selecting symbolsdiscretely in time and/or discretely in a frequency. By this means, thetime diversity effect and/or the frequency diversity effect can beobtained, so that it is possible to further improve received quality ofpartial data and further enhance the improvement effect in receivedquality through retransmission.

Furthermore, in the above-described embodiment, the case has beendescribed where LDPC codes are used alone, but the present invention canbe also applied to a case where concatenated codes which use LDPC codestogether with other error correcting codes are used.

Other Embodiment

In the above-described embodiment, the case has been described where, bytransmitting partial data of the LDPC encoded data from fewer antennasupon retransmission than upon the last transmission, uponretransmission, transmission using a transmission method with a higherdiversity gain than upon the last transmission is realized, but thetransmission method with a high diversity gain carried out uponretransmission is not limited to the above-described embodiment, and,for example, space-time block codes and Cyclic Delay Diversity may alsobe used as the transmission method for obtaining a high diversity gain.Moreover, it is more effective to decrease the M-ary number for eachretransmission time.

Here, the case will be described where space-time block codes and CyclicDelay Diversity are applied to retransmission.

FIG. 10 shows an example of the transmission method when space-timeblock codes are used. From transmission antenna 901, symbol S1 istransmitted at time t, and symbol −S2* is transmitted at time t+1. Fromtransmission antenna 902, symbol S2 is transmitted at time t, and symbolS1* is transmitted at time t+1. Here, * indicates a conjugate complexnumber. In this way, by using space-time block encoded signals,respective symbols S1 and S2 can be combined at a maximum ratio uponsignal separation regardless of transmission path fluctuations h1(t) andh2(t), so that it is possible to obtain a large coding gain and adiversity gain. Therefore, the received quality of the retransmissiondata—the error rate performance—can be improved. In this way, bytransmitting data to be retransmitted using space-time block codes, itis possible to obtain an effect equivalent to or higher than that ofreducing the number of transmit antennas upon retransmission.

FIG. 11 shows a data flow between the base station and the terminal inthis case. The difference between FIG. 11 and FIG. 7 is thatretransmission data 2A and P1, retransmission data 2B and P1,retransmission data 3A and P1, retransmission data 3B and P1,retransmission data 3A and P2 and retransmission data 3B and P2 aretransmitted using modulated signal A and modulated signal B.

FIG. 12 shows a configuration example of a multi-antenna transmissionapparatus in the case of transmitting partial data of LDPC to beretransmitted using space-time block codes. In FIG. 12 in whichcomponents corresponding to those in FIG. 2 are assigned the samereference numerals, the difference between multi-antenna transmissionapparatus 1100 and multi-antenna transmission apparatus 100 in FIG. 2 isthat modulation section 1101 is shared among channels so as to allowspace-time block encoding. When inputted frame configuration signal 117indicates that transmission is not retransmission, modulation section1101 outputs serial signals 107A and 107B of modulated signal A andmodulated signal B without performing space-time block encoding. On theother hand, when inputted frame configuration signal 117 indicates thattransmission is retransmission, modulation section 1101 performsspace-time block encoding and outputs serial signals 107A and 107B ofmodulated signal A and modulated signal B.

Upon receiving such space-time block codes, when control information 319indicates retransmission data, signal processing section 311 ofmulti-antenna reception apparatus 300 (FIG. 4) performs signalprocessing for separating space-time block codes and outputs basebandsignal 312A of modulated signal A and baseband signal 312B of modulatedsignal B.

Here, the case has been described as an example where space-time blockcodes are transmitted using two transmit antennas, but the space-timeblock codes are not limited to the above-described ones, and, even in acase where transmission is performed using three or more transmitantennas, space-time blocks corresponding thereto may be used.Furthermore, the case has been described as an example where encoding isperformed on the time axis, but encoding is not limited to theabove-described explanation, and in the case of a multicarrier schemesuch as OFDM, encoding may also be performed using the frequency axis.For example, it is also possible to transmit S1 with carrier i and −S2*with carrier i+1 at time t from transmission antenna 901 and transmit S2with carrier i and S1* with carrier i+1 at time t from transmissionantenna 902.

Next, an example of Cyclic Delay Diversity will be explained using FIG.13.

FIG. 13 shows an example of the frame configuration when Cycled DelayDiversity is performed using 12 symbols. The signal transmitted atantenna 114A in FIG. 2 is transmission signal A in FIG. 13, and thesignal transmitted with antenna 114B in FIG. 2 is transmission signal Bin FIG. 13. As for transmission signal A, S1, S2, . . . , S11, S12 aretransmitted at times i+1, i+2 . . . , i+11, i+12, respectively.Transmission signal B has the frame configuration shifted by a giventime period with respect to transmission signal A. Here, S7, S8, . . . ,S5, S6 are transmitted at times i+1, i+2, . . . , i+11, i+12,respectively. By adopting such a frame configuration, a diversity gaincan be obtained on the reception apparatus side by equalizing thereceived signals, so that it is possible to improve received quality ofsignals S1 to S12 and improve the error rate characteristic of data.That is, by using Cycled Delay Diversity upon retransmission of thepartial data of LDPC, it is possible to improve received quality ofretransmission data and thereby improve the overall error rateperformance of the received data.

Furthermore, in the above-described embodiment, the case has beendescribed where different LDPC encoded data is transmitted from twoantennas upon the first transmission, and part of the LDPC encoded datais transmitted from one antenna upon retransmission, but the presentinvention is not limited to this.

For example, a case will be considered as an example where in amulti-antenna communication system using three transmit antennas (basestation) and three receive antennas (terminal) as shown in FIG. 14,transmission signals Txa(t), Txb(t) and Txc(t) are transmitted fromtransmit antennas 1401, 1402 and 1403, respectively. At this time, whenit is assumed that received signals received at receive antennas 1404,1405 and 1406 are R1(t), R2(t) and R3(t), respectively, the followingrelational equation holds.

$\begin{matrix}\lbrack 3\rbrack & \; \\{\begin{pmatrix}{R\; 1(t)} \\{R\; 2(t)} \\{R\; 3(t)}\end{pmatrix} = {\begin{pmatrix}{h\; 11(t)} & {h\; 12(t)} & {h\; 13(t)} \\{h\; 21(t)} & {h\; 22(t)} & {h\; 23(t)} \\{h\; 31(t)} & {h\; 32(t)} & {h\; 33(t)}\end{pmatrix}\begin{pmatrix}{{Txa}(t)} \\{{Txb}(t)} \\{{Txc}(t)}\end{pmatrix}}} & ( {{Equation}\mspace{14mu} 3} )\end{matrix}$

The base station transmits data other than retransmission data usingthree transmit antennas. Here, when partial data is retransmitted, thepartial data can be transmitted using a transmission method having ahigh diversity gain by applying space-time block codes and Cyclic DelayDiversity and moreover retransmitting the partial data with threeantennas 1401, 1402 and 1403.

As another method for transmitting partial data using a transmissionmethod with a high diversity gain, there is a method of transmittingsignals Txa and Txb as retransmission signals (partial data) using twotransmit antennas 1401 and 1402 as shown in FIG. 15. In this way, theterminal can receive signals using three receive antennas 1404, 1405 and1406, so that it is possible to obtain higher diversity gains than whentransmission is not retransmission. In this way, the method of improvingthe diversity gain upon retransmission by reducing the number oftransmit antennas has a merit of being more excellent than space-timeblock codes and Cyclic Delay Diversity in the data transmission speed.That is, by transmitting N lines of transmission signals through Ntransmit antennas when transmission is not retransmission, andtransmitting retransmission data with M (M<N) lines of transmissionsignals through the M transmit antennas only upon retransmission, it ispossible to obtain a high diversity gain upon retransmission withoutconsiderably reducing the data transmission speed.

Furthermore, in the above-described embodiment, the case has beendescribed as an example where the present invention is applied to theradio communication system based on an OFDM scheme which is one ofmulticarrier schemes, but the present invention is not limited to this,and similar effects can also be obtained when the present invention isapplied to a radio communication system based on a single carrier schemeor a radio communication scheme based on a spread spectrum scheme.

The present application is based on Japanese Patent Application No.2004-340371, filed on Nov. 25, 2004, entire content of which isexpressly incorporated by reference herein.

INDUSTRIAL APPLICABILITY

The present invention can be widely applied to a multi-antennacommunication system which performs radio transmission using a pluralityof antennas such as an OFDM-MIMO communication system.

The invention claimed is:
 1. A transmission apparatus comprising: anencoding circuit configured to encode data according to a parity checkmatrix of Low Density Parity Check coding to generate an encoded datasequence such that the encoded data sequence includes a first bit and asecond bit, the parity check matrix having a first column and a secondcolumn to generate the first bit and the second bit, respectively; and atransmitting circuit configured to transmit the encoded data sequenceand configured to retransmit the first bit without retransmitting thesecond bit, wherein a first number of 1 bits in the first column isgreater than a second number of 1 bits in the second column.
 2. Areception apparatus comprising: a receiving circuit configured toreceive: an encoded data sequence; and a first retransmitted bit whichis identical to a first bit included in the encoded data sequence, asecond bit in the encoded data sequence having not been retransmitted;and a decoding circuit configured to decode the encoded data sequenceaccording to a parity check matrix of Low Density Parity Check coding togenerate decoded data, the parity check matrix having a first column anda second column to generate the first bit and the second bit,respectively, wherein a first number of 1 bits in the first column isgreater than a second number of 1 bits in the second column.